The recording material is generally that which is obtained by forming a thermally color-forming thermal recording layer on a substrate, and when it is heated with a thermal head, a hot pen, a laser beam, etc., an electron-donating compound that is a dye precursor and an electron-accepting compound that is a color developer react instantly with each other to give a recorded image. The above thermal recording material has advantages that a recording is obtained with a relatively simple device, that the maintenance of the device is easy and that no noise is made. Such thermal recording materials are therefore used in broad fields including a measuring meter, a facsimile machine, a printer, a computer terminal, a label printing machine, a ticket or card-issuing machine, and the like.
Further, as these thermal recording materials have come to be used in a variety of fields, there are some use fields where printing is made thereon. In recent years, there is commercially strongly demanded a thermal recording material having strength suitable for printing and having a protective layer that is so excellent in a barrier property as it can keep a printing solvent from developing a color on a ground like ground fogging.
Further, due to recent developments of recording systems in particular, thermal recording materials have come to be used in more severe environments, and in particular when they are used in environments where they are exposed to adhering water, there are strongly demanded thermal recording materials having protective layers excellent in water resistance.
Protective layers having various constitutions are proposed for the purpose of imparting thermal recording materials with surface strength suitable for printing, improving the water resistance thereof and further enabling stable production. As an example of methods therefor, JP 2000-15932 proposes using a diacetone-modified polyvinyl alcohol as a binder and a hydrazine compound as a crosslinking agent in a protective layer and further adding a water-soluble organic amine for inhibiting an increase in the viscosity of a coating liquid and obtaining a highly stable coating liquid. Since, however, the presence of the organic amine also inhibits that reaction between the binder and the crosslinking agent which is necessary for attaining water resistance, the water-resistance-achieving speed of the protective layer is delayed. Further, Japanese Patent No. 3716736 proposes incorporating a diacetone-modified polyvinyl alcohol as a binder into a protective layer and incorporating a hydrazide compound as a crosslinking agent into a thermal recording layer. However, the protective layer is insufficient in attaining water resistance, and the problem is that the viscosity of a coating liquid for a thermal recording layer is increased, that the hydrazide compound inhibits the color development of the thermal recording layer, and that there is caused ground fogging which is the development of a color at the stage of the coating liquid. Further, JP 2002-283717 proposes incorporating into a protective layer a polyvinyl alcohol having a reactive carbonyl group as a binder, a hydrazide compound as a crosslinking agent and a basic filler for improving the stability of a coating liquid for the protective layer. Since, however, the basic filler has no effect on the promotion of a reaction between the polyvinyl alcohol and the crosslinking agent, the water-resistance-achieving speed of the protective layer is slow. Therefore, the present state is that there has not been obtained any thermal recording material that ensures the keeping of coating liquid stability and that exhibits sufficient surface strength, the fast speed of achieving water resistance and high water resistance.